Grocery Spending for a Family of 3 in 2010

Our average spending per month was $320.90. Yet these official figures from the United States Department of Agriculture indicate that, in 2008, a five-year-old child and two parents in their thirties could expect to spend $422.10 per month if they're "thrifty". Our pantry is close to full most of the time, we all eat hearty snacks as well as meals, we make lots of food to share with friends as well as donating to the food pantry, and we're generally very happy with our diet and don't feel we're scrimping. How do we do it on so low a budget?